Abstract: The application of frequency converters (VDCs) is now widespread in domestic factories and mines; they are almost ubiquitous wherever electric motors are used. A VDC is an organic combination of high-voltage and low-voltage electricity, and of hardware and software. Its powerful functions, comprehensive detection and protection circuits, intelligent and flexible control, and the non-standard and specialized requirements of its electrical components necessitate unique troubleshooting approaches and methods. The application and widespread use of VDCs and PLCs, among other industrial control equipment, have even led to the development of a specialized industry for their maintenance, becoming a branch of electrical technology. This has also profoundly changed the concept of electrical engineering. This paper mainly analyzes the inverter module and drive circuit of a general-purpose VDC to understand the common forms and characteristics of IGBT faults, meeting the needs of solving practical maintenance problems.
I. Characteristics of IGBT Inverter Modules and Drive Circuits
IGBT devices are generally considered voltage-controlled devices, requiring only a certain level of excitation voltage without needing to draw excitation current. This is because there is a junction capacitance between the gate and emitter of the IGBT. The turn-on and turn-off processes essentially involve charging and discharging this junction capacitance, generating a peak current. Furthermore, the IGBTs in the inverter output circuit operate under pulses of several kHz, and their gate bias voltage is also a pulse voltage of several kHz. Capacitors have the characteristic of passing AC and blocking DC; compared to pulse voltages of tens of kHz, the capacitive reactance is relatively small, resulting in a large charging and discharging current. Therefore, based on the above analysis, it can be concluded that the IGBTs used in the inverter output circuit should be current- or power-driven devices, not purely voltage-controlled devices. The output stage of the drive circuit (Note 1) should also be a power amplifier circuit. This is because driving an IGBT consumes a certain amount of power and outputs a certain amount of current. Therefore, high-power IGBT modules need to be driven by a power amplifier circuit.
Therefore:
(1) To enable the IGBT to turn on quickly, and given that the time for providing the positive gate bias voltage is very short, it is necessary to provide the largest possible drive current (charging current) to ensure that the IGBT turns on quickly and reliably. A certain saturation depth is also required during conduction to reduce conduction losses. However, a higher Uce is not always better; generally, +10~15V is chosen. The same applies to the cutoff control; the drive circuit must quickly release the charge on the gate-emitter junction capacitor. To ensure reliable IGBT cutoff, a sufficiently strong cutoff negative voltage must be provided to meet the IGBT turn-off requirements. A reverse gate voltage (generally -5~-15V) should also be applied to the IGBT in the cutoff state to ensure reliable cutoff even when switching noise occurs at the gate.
(2) The basic requirements for the drive circuit are relatively high, and it should have the following basic performance characteristics: 1. Strong dynamic driving capability, capable of providing drive pulses with steep leading and trailing edges to the IGBT gate. 2. The internal resistance of the driver should not be too small, so as to prevent stray inductance of the circuit from forming underdamped oscillations with the gate capacitance. 3. Sufficient input and output electrical isolation capability. As a closely related part of the inverter circuit, the drive circuit has a huge impact on the three-phase output of the frequency converter. The performance and quality of the drive circuit are important conditions for ensuring the normal operation of the IGBT module.
II. Operating Characteristics and Protection of IGBT Inverter Modules
IGBT modules operate under high voltage, high frequency, and high current conditions. These modules feature an inverter frequency of up to 18kHz, fast dynamic response, and good stability. They combine the advantages of high input impedance of MOSFETs and low on-state voltage drop of GTRs.
During turn-on, the PNP transistor operates as a MOSFET for most of the time. However, in the later stages of the Uce decrease process, the PNP transistor transitions from the amplification region to the saturation region, adding a delay time and causing the Uce waveform to split into two segments. During turn-off, because the charge stored in the PNP transistor cannot be quickly eliminated, the current Ic also splits into two segments. Within most of the IGBT's conduction current range, Ic and Uge have a linear relationship; the larger Uge is, the larger Ic is.
The IGBT inverter module in the inverter output circuit operates under pulses at a frequency of several kHz, and its gate bias voltage is also a pulse voltage of several kHz. The operating conditions of IGBTs are harsh, requiring reliable turn-on and rapid turn-off. With the continuous development of power electronics technology, the application of IGBT inverter bridges in energy conversion circuits is becoming increasingly widespread. Therefore, their protection is extremely important.
Among the various factors determining the reliability of IGBT operation, the overcurrent protection circuit plays a crucial role. Since IGBTs have limited capacity to withstand overcurrent or short circuits, the overcurrent protection circuit not only affects the performance and operational safety of the IGBT module itself but also impacts the performance and safety of the entire system. Generally, the forward voltage drop (Uce) of the IGBT is detected to reflect the magnitude of the conduction current. This method is not only simple but also prevents IGBT desaturation during operation. The forward saturation voltage drop of an IGBT is approximately 2.5–3V; generally, when Vce exceeds 7V, it is considered an overcurrent. If it is a momentary overcurrent, the protection circuit does not handle it; however, if the overcurrent duration is slightly longer or the overcurrent amplitude is larger, a protective shutdown will be implemented.
III. Troubleshooting and Analysis of IGBT Inverter Modules
(I) In our daily production and use, IGBT module failure is a common fault phenomenon, and the causes of failure can be varied. Below are a few examples related to the drive circuit when repairing frequency converters:
(1) A Yaskawa 616G5 3.7KW inverter had a fault where the three-phase output was normal, but the motor vibrated at low speeds, making normal production impossible. The analysis indicated that the inverter's drive circuit was damaged. After confirming the fault, the inverter was turned on, and the IGBT inverter module was removed from the circuit board. An electronic oscilloscope was used to observe whether the waveforms of the six drive circuits were consistent when they were turned on. The inconsistent drive circuit was identified, and the IC on that drive circuit was replaced. It is usually a PC923 or PC929. If the inverter has been used for more than 3 years, it is recommended to replace all the electrolytic capacitors in the drive circuit. After observing with an oscilloscope, the IGBT inverter module was installed after the six waveforms were consistent, and a load test was performed. The vibration phenomenon was eliminated.
(2) Hitachi VWS5.5HF3EH inverter, after starting, emits a "squeaking" sound. The sound gradually becomes sharper and more piercing as the frequency increases. After about 5 seconds, the inverter issues an "OC" fault alarm, which is also the case under no-load. Since the no-load alarm is an overcurrent alarm, it is not a true overcurrent fault. Therefore, the IGBT module is suspected to be faulty. However, no problem is found after inspection. The main circuit is then suspected to be the problem. The large capacitor leakage generates large pulsations, causing overcurrent. The capacitance and discharge performance of the capacitor are checked, but no problem is found. Finally, the drive circuit is suspected to be the problem. The overcurrent protection does not apply when the IGBT module is not connected, but it does apply when the IGBT module is connected. With all drive signals normal, the faults are: first, the drive module performance is degraded and cannot drive the IGBT; second, the capacitor failure causes unstable drive voltage and abnormal drive signal, resulting in poor IGBT switching timing and causing overcurrent in the main circuit. Since the capacitor is a vulnerable component, the capacitor is checked first. All the capacitors are removed and checked. The leakage resistance of the capacitors is basically the same, and the charging and discharging are normal. When measuring with a universal bridge, it was found that the capacitance of one capacitor was smaller than the others, but the difference was not significant, so it was not replaced. The three-phase drive module was then replaced, but the fault persisted. The drive module fault was thus eliminated. Since the possibility of simultaneous failure of all three phases is relatively small, the method of swapping identical components between the normal and faulty circuits was used to find the faulty component. Finally, the cause of the fault was found: the problem was still with the capacitor with the smaller capacitance. The insufficient capacitance caused the drive signal to malfunction, resulting in overcurrent in the machine. After replacing the capacitor, the inverter returned to normal operation.
(3) Delta VFD075-M inverter, the fault phenomenon is arcing at the output of the inverter. After disassembly and inspection, it was found that the IGBT inverter module was shorted and the drive circuit board was severely damaged. The correct solution is to first remove the damaged IGBT inverter module. When disassembling, the main thing is to protect the board from secondary damage by human. Replace the damaged electronic components on the drive circuit one by one and connect the open circuits on the board with wires (pay special attention to scraping the burnt parts clean to prevent arcing again). When the resistance and voltage of the six drive circuits are the same, use an oscilloscope to measure the waveform. However, the inverter reports an OCC fault as soon as it is turned on (the Delta inverter will alarm when it is turned on without an IGBT inverter module). Use a light bulb to connect the P1 of the module to the board, and connect the others with wires. It still trips OCC when restarted. It is determined that there is still a problem with the drive circuit. Replace the optocouplers one by one. It was found that the optocouplers of the drive circuit have a detection function. One of the optocouplers has a damaged detection function. After replacing it with a new one, the start is normal.
(4) Fuji G9 inverter, the fault is that there is no display when powered on. The analysis is that the inverter switching power supply is damaged. The inverter was opened and the switching power supply line was checked, but the switching power supply components were not damaged. There was no display when DC power was applied to the positive and negative DC terminals. At this time, it should be estimated that there may be a problem with the drive. All the capacitors in the drive circuit were removed and some capacitors were found to be leaking. After replacing them with new electrolytic capacitors, the inverter worked normally after being powered on again.
(5) VACON132CX4 (process and cause of scrapping): After 3 years of use, it worked normally in summer, but sometimes failed to start in winter, reporting memory errors (EPPROM, ERROM). At this time, the machine was powered on, the screen was lit, and the fan was running. After powering off and restarting, the fault disappeared. Considering that the working environment of this inverter was poor, with waste steam discharged nearby, and it was winter with the indoor temperature below 20 degrees Celsius, there was dust on the printed circuit board inside the machine, which could cause condensation and lead to leakage of IC pins. Especially for surface-mount ICs such as memory and CPU, the pins are densely packed, and condensation can cause abnormal pin levels, so it is not surprising that the fault is reported. After waiting a while and powering off and restarting, the fault disappeared because the internal fan of the machine worked, blowing away the condensation, and the heat reduced the water vapor. Powering off also served as a fault reset function. In the 4th year, this inverter began to frequently report radiator overheating faults. A lot of dust was blown out with a dust gun, and the machine would be fine after cooling down and restarting. In the summer of the 5th year, not only did it report overheating faults, but it also reported overcurrent faults. After thorough dust removal, overheating issues decreased somewhat, but overcurrent remained largely unchanged. The machine started and stopped intermittently, eventually reporting an "INRERTER FAULT" error, indicating an inverter failure, and could no longer be started. Disassembly revealed a burnt-out drive board, two of the six IGBT inverter modules were blown, and the control terminals of the unblemished modules were covered in a sticky, black grime. A multimeter on the 100K range showed an unstable leakage resistance of approximately 30K ohms between the collector and emitter terminals; all four modules showed the same issue. The grime was cleaned with anhydrous alcohol and dried with a hairdryer. Measurements of the control gate and the collector/emitter terminals showed infinite resistance. Resistance between other electrodes was normal. The internal resistance diodes were normal, and the gate-emitter capacitance was measured at 2000pp, showing good consistency. All four modules were functioning correctly. The inverter was beyond repair due to severe damage and had to be scrapped.
